1. Field of the Invention
The present invention relates to a general-purpose multi-wavelength lens used in a recording and reproducing apparatus of a multi-wavelength optical system using a plurality of kinds of monochromatic light which is compatible with optical storage medium of different types such as compact discs (CD) including CD-R, digital versatile discs (DVD), and Blue-laser-based advanced optical discs (AOD), and a multi-wavelength optical system, an optical head and an optical disc apparatus.
2. Related Background Art
Conventionally, compatible optical disc apparatus capable of reproducing different types of optical discs such as CD and DVD have been proposed. The CD and DVD (which will be hereinafter collectively referred to as an optical disc) have a transparent substrate, one side of which has an information surface. The optical disc is composed of two transparent substrates combined together with their information surfaces facing each other, or composed of a transparent substrate and a transparent protection substrate with the information surface of the transparent substrate facing the protection substrate. To reproduce information signals stored in these optical discs, the optical disc apparatus condenses a laser beam from a light source on the information surface of the optical disc through the transparent substrate. As detailed later, wavelengths of the laser beam used for CD and for DVD are different. The optical disc apparatus uses an objective lens for condensing the laser beam. The thickness of the transparent substrate provided with the information surface differs according to a type of the optical disc (a difference in a laser beam wavelength). While the transparent substrate of CD is 1.2 mm in thickness, that of DVD is 0.6 mm. For the optical disc apparatus to reproduce optical discs of different types, it is required to condense a laser beam on the information surface even if the thickness of the transparent substrate differs with the type of the optical disc. Besides, a new optical disc apparatus using Blue-laser of approximately 400 nm wavelength for reproducing information is recently proposed. Thus, it is desirable for the optical disc apparatus to be compatible with the new optical disc in addition to CD and the existing DVD.
One approach for the above optical disc apparatus is to provide a pickup with objective lenses for different types of optical discs to change the objective lenses in accordance with the type of the optical disc in use, or to provide pickups for different types of the optical discs to change the pickups in accordance with the type of the optical disc in use. However, for miniaturization and cost reduction, it is preferred to have a single objective lens usable for any types of the optical disc.
Known as this kind of objective lens is one disclosed in Japanese Unexamined Patent Application Publication No. H09-145995, for example. A lens surface of the objective lens disclosed therein is radially sectioned into more than two loop zones, and every other loop zonal lens surfaces and the other every other loop zonal lens surfaces are different in refracting power. For laser beams of the same wavelength, the every other loop zonal lens surfaces condense the laser beams on the information surface of the optical disc (DVD) having a thinner transparent substrate (0.6 mm), and the other every other zonal lens surfaces condense the laser beams on the information surface of the optical disc (CD) having a thicker transparent substrate (1.2 mm), for example.
Another example is one disclosed in Japanese Unexamined Patent Application Publication No. 2000-81566 (U.S. Pat. No. 6,118,594). It discloses the optical disc apparatus using a laser beam of a shorter wavelength (635 nm or 650 nm) for DVD having a thinner transparent substrate while using a laser beam of a longer wavelength (780 nm) for CD having a thicker transparent substrate. The optical disc apparatus is provided with an objective lens used in common for the two kinds of laser beams. The objective lens has a diffractive lens structure where a plurality of minute loop zonal steps are thickly formed on one side of a refractive lens having a positive refractive power. The diffractive lens structure is designed to condense, on the information surface, diffracted light of laser beams having a shorter wavelength for DVD with a thinner transparent substrate and diffracted light of laser beams having a longer wavelength for CD with a thicker transparent substrate. Further, the lens is designed to condense diffracted light having the same diffractive order for both of the diffracted light. The laser beam having the shorter wavelength is used for DVD because a storage density of DVD is higher than that of CD, thus requiring a smaller beam spot. As well known, the diameter of an optical spot is proportional to a wavelength and inversely proportional to a numerical aperture (NA).
There is also disclosed an objective lens of a loop zonal phase correction lens type having a loop zonal phase shifter on the lens surface in Japanese Unexamined Patent Application Publication No. 2001-51192. In this lens, a lens surface designed to have no wavefront aberration for a laser beam having wavelength λ1 of 640 nm is first set as a reference. A surface of the objective lens is then radially sectioned into a plurality of loop refractive zones which are formed to have predetermined steps (i-th step from a center of the lens is referred to as di) from the reference lens surface. Due to the step di, each of the refractive surfaces allows the laser beam of DVD to phase-shift by integral multiple mi of the wavelength λ1 against the reference lens surface, thereby reducing wavefront aberration in a CD system.
The above conventional techniques allow a common objective lens to be used for both DVD and CD. This eliminates the need for changing members including objective lenses for each use of DVD or CD, which is effective in reducing costs and simplifying the structure. However, the technique disclosed in Japanese Unexamined Patent Application Publication No. H09-145995 uses different loop zonal lens surface of the objective lens for DVD to CD, causing a large area to remain inactive for incident laser beams, which extremely lowers light use efficiency.
Further, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-81566 (U.S. Pat. No. 6,118,594) has a problem that diffractive efficiencies for different wavelengths are unable to become 100% at the same time because it uses the diffracted light by the diffractive lens structure. In this diffractive lens, for a laser beam having the shorter wavelength (635 nm or 650 nm) used for DVD and for a laser beam having the longer wavelength (780 nm) used for CD, the diffractive efficiency is designed to become 100% at intermediate wavelength between the two in order to make the diffractive efficiency well balanced for the laser beams in use. Besides, the technique requires minute steps to be formed on the lens surface to make the diffraction lens structure, which is vulnerable to processing error. If the diffractive structure fails to be formed as designed, it causes a decrease in the diffractive efficiently. When the diffractive efficiency decreases or when it does not reach 100%, it is unable to condense all the incident light on the information surface formed on the transparent substrate of the optical disc, which results in the light loss.
Further, the loop zonal phase correction lens type disclosed in Japanese Unexamined Patent Application Publication No. 2001-51192 has the following problem. This conventional technique sets the lens surface designed to have no wavefront aberration for the DVD laser beam as a reference surface, and forms a refractive surface lower than the reference surface by the height of a step di, which is integral multiple mi of the wavelength λ1 of the DVD laser beam so as to reduce the wavefront aberration for the CD laser beam. Further, this technique designs the curved surface structure of each loop zone to prevent a focal point in each loop zone from shifting due to the formation of the step on the lens surface. Though, however, this technique allows sufficient decrease in the wavefront aberration for DVD, it does not achieve sufficient decrease in the wavefront aberration for the CD laser beam.
Many conventional arts including “Principle of Optics”, The Optronics Co., Ltd., Nov. 26, 1990, p.198 describe that the appropriate value of RMS wavefront aberration is 0.07 λRMS and below, according to Marechal criterion. Japanese Unexamined Patent Application Publication No. 2001-51192 mentioned above also discloses the embodiment to suppress the wavefront aberration to 0.07 λRMS or less. In optical disc apparatus, however, the value of 0.07 λRMS or less should be a target value of the optical disc apparatus as a whole, and this value is still not sufficiently low for a target value of a single objective lens. Since an optical disc as a whole has various factors to undesirably increase the RMS wavefront aberration, such as astigmatic difference of a laser, aberration of a collimator lens, aberration of a reflecting mirror and a transmitting mirror, and tilt displacement of an optical pickup and an optical disc, the RMS wavefront aberration should be as low as possible, not just 0.07 λRMS or less, in a single objective lens. Specifically, the RMS wavefront aberration for a single objective lens is preferably 0.035 λRMS or less, more preferably 0.030 λRMS or less, and further preferably 0.025 λRMS or less. In Japanese Unexamined Patent Application Publication No.2001-51192, the RMS wavefront aberration is 0.001 λRMS for DVD and 0.047 λRMS for CD in the first embodiment, and it is 0.019 λRMS for DVD and 0.037 λRMS for CD in the second embodiment. The suitable wavefront aberration is achieved for DVD, but not for CD, which still exceeds 0.037 λRMS. This technique thus fails to achieve sufficiently low wavefront aberration for both DVD and CD.